I fellow Engineer (Aerospace Quality Engineer) was telling me the other day not to mix Titanium with Aluminium as they react with eack other and corrode. Don't know how true this is, only found out on Thursday._________________2000 Manual 996 C4 Arctic Silver Convertible

Bluebird911Nürburgring

Joined: 29 May 2010Posts: 417

Posted: Sat Apr 14, 2018 10:09 pm Post subject:

alex yates wrote:

I fellow Engineer (Aerospace Quality Engineer) was telling me the other day not to mix Titanium with Aluminium as they react with eack other and corrode. Don't know how true this is, only found out on Thursday.

Your fellow engineer is correct Alex.

The Galvanic Series Chart explains that the "anodic" or "less noble" metals at the negative end of the series such as magnesium and aluminium will be subjected to bi metal galvanic corrosion when in contact with "cathodic" or "noble" end of the series materials such as gold or titanium, in the presence of an electrolyte. Salt water is a particularly effective electrolyte, so consideration of metal combinations is particularly important in Marine engineering for example. Better to choose material combinations that are closer together in the series and stainless steel is a good example of this. The potential (voltage) difference of Al v Stainless Steel is much smaller than Al v Ti. Heat also promotes this galvanic corrosion

That is the theory out of the way. An electrolyte has to be present for corrosion to take place, and would suggest that the screws into the head is potentially dry, though any dampness in there will cause corrosion and seize the stud. This is more likely to happen with a Ti stud. In theory, it should also accelerate corrosion around the stainless steel flanges / fasterners.

Yep - I totally get all that and understand the Chemistry of it (working in Cathode manufacturing for TVs and also spark and wire eroding in the Toolroom). Usual case of good stuff used in the wrong application with regards to ti and Ally.

My Stainless bolts have now been in there 3.5 years but look like they were fit last week._________________2000 Manual 996 C4 Arctic Silver Convertible

As a side note, I have Titanium implants in the front of my face. The pain when they drill and tap into the bone is purely excruciating........like a teet I had the operation done whilst conscious

They use Titanium because the bone fuses with the metal and locks it in place.

Its ability to physically bond with bone gives titanium an advantage over other materials that require the use of an adhesive to remain attached. Titanium implants last longer and much higher forces are required to break the bonds that join them to the body compared to their alternatives.

I hope I don't have to have mine removed.......that will be excruciating!!!! _________________2000 Manual 996 C4 Arctic Silver Convertible

DammitKyalami

Joined: 23 Sep 2016Posts: 1842

Posted: Sun Apr 15, 2018 8:55 am Post subject:

I've got titanium Rawl plugs in my jaw as well, did they tell you how it was discovered that bone bonds onto Ti?

It was a researcher who was working with Rabbits - something to do with blood chemistry IIRC. Anyway, the experiment (for a specific rabbit) was over so the researcher decided to "recover" the titanium capsule that was inside the rabbit - and made the discovery that the rabbits bones had bonded to it.

I know I also had a bone graft as a lot of the bone had rotted away. They used cow bone to do the graft then cover it in pig skin while it knits, stitching the flesh up over the top. Then 3 months later, open me up and remove the skin._________________2000 Manual 996 C4 Arctic Silver Convertible

Bluebird911Nürburgring

Joined: 29 May 2010Posts: 417

Posted: Sun Apr 15, 2018 1:24 pm Post subject:

wasz wrote:

Yes pure titanium is more reactive than stainless.

However both stainless steel and titanium form a passive oxide layer when freshly cut, it is this that protects the metal and resists corrosion.

Titanium's oxide layer is more protective than the layer that forms on stainless. This oxide layer also insulated the titanium from other metals in contact.

This is why medical implants use titanium, it is least prone to corroding because the inert oxide layer protects the rest of the metal.

Hi Wasz,

You are correct about the oxidation layers and they form a protection to environment conditions, but these layers are not an electrical insulator, so will not prevent bi-metal corrosion when the joint between the metals contains an electrolyte. Remember, bi-metal corrosion is an electrical process.

If oxidation layers prevented galvanic corrosion, there wouldn't be the sophisticated inhibitors or sacrificial annodes that have been developed to manage this type of corrosion.

Because of its good oxidation corrosion properties, titanium is a really good material for use in the body such as the application you suggest. Ti is not used in bodies because its oxide layer prevents galvanic corrosion, this is not true. When in the body, Ti will not come into contact with another metal via an electrolyte; unless you are Iron Man!!!

Cue Black Sabbath!!

Alex, I recall studying this stuff at Uni many years ago, but it appears you have practical experience of the principles!! However, I have seen and worked on some ropey early Land Rovers (Aluminium body panels on Steel chassis constructed using steel fastners) before the motor industry took bi metal corrosion seriously!!

X51 996Trainee

Joined: 14 Nov 2016Posts: 89Location: Surrey

Posted: Sun Apr 15, 2018 1:52 pm Post subject:

It seems that no matter what type of fasteners are used there’s a downside!

Isn’t stainless lower in strength than the regular steel bolts used? And what about the repeated heat cycling?

Always a hassle with steel/alloy fixings on these cars - which is pretty much everywhere, and as they’re getting older there’s more and more trouble waiting for the future.

Have to say the quality of the factory fasteners can’t be all that. I’ve worked on loads of cars over the years - many much older than 996s and have never seen such badly rusted exhaust fixings as you see on these.

What are the typical exhaust studs used on most alloy heads made of? Black steel or coated steel with the copper/copper coated nuts - never had any trouble with removing either the nuts or the studs even on cars twice the age and mileage of most 996s.

I think going back to the OP’s question - personally I’d leave alone. There’s a good chance you’ll never need to remove the manifolds in situ, and if you do it now you’re still going to have hassles anyway. Save the cash towards other contingency work or something

The problem with a 911 is the engine's almost scraping along the floor hanging out the back end and subject to everything off the road unlike most other cars._________________2000 Manual 996 C4 Arctic Silver Convertible

I have a pair of old manifolds that were blowing that I have cut the pipes off - they make a perfect drilling jig. Feel free to PM me is you (or anyone) wants to use them for such!

Gooey36Trainee

Joined: 23 Aug 2017Posts: 81

Posted: Mon Apr 16, 2018 8:28 pm Post subject:

I’ve bitten the bullet as the cars a keeper for me and it’s booked in with my local Porsche specialist tomorrow for drilling/helicoiling worse case. It does have a slight blow on it though...The rest of the exhaust is new so the old rusty manifold just looks ‘orrible as well.

I had/have opted for Ti fixings over SS given the threads in the head should be “dry” and a galvanic cell will not be created. Any residual moisture in the heads should disappear with heat from the head I would’ve thought. I do have some tef-gel which we use at work to prevent galvanic corrosion - not sure I need it though

I’ve used Ti fixings in many road and track sports bike engine applications in the past and some car applications and never had any galvanic corrosion issues in the past. Have had a few stainless fasteners sheer though!

i have been thinking of stripping mine also, at the moment no leaks, just know from prior experience they will be a bas.'#d, has anybody the jig for hire, they are expensive for one time use, regards to all

rdodgerMonza

Joined: 08 Apr 2018Posts: 150Location: Cheshire

Posted: Tue Apr 17, 2018 7:51 am Post subject:

alex yates wrote:

Kool.

I know I also had a bone graft as a lot of the bone had rotted away. They used cow bone to do the graft then cover it in pig skin while it knits, stitching the flesh up over the top. Then 3 months later, open me up and remove the skin.

This deserves it's own thread!

I have 2 Ti implants. I also had a bone graft but from my hip. They did say at first they would use bone from my skull form the non dominant side. I thought the hip would be the safer option.

I too had mine done under sedation rather than GA. Not fun!

Gooey36Trainee

Joined: 23 Aug 2017Posts: 81

Posted: Tue Apr 17, 2018 9:14 am Post subject:

P911X50 wrote:

Hi Gooey36,

Mine a keeper too and I am also considering getting it done. Has your specialist indicated likely costs??

Thanks

Yes - looking at £37 a bolt to remove and helicoil. Most I rang local to me (Notts) wanted between 350-700 worst case.

Another newbie here - owner of a 996 C2 3.6 manual since October 2017!

Already done the manifold to cat bolts. Fortunately they had been replaced with nuts and bolts previously rather than the press fit studs which are a challenge to remove. I've just used steel but purchased 3 sets so they will become a service item.

I plan to do the manifold bolts November time, when it will see a lot less use. I've already purchased poppopbangbang's TI stud set and picked up a Stomski jig from ebay. Thinking is if I have the kit to drill them out, they'll all come out easy!

Plan is heat the bolts up with a blow lamp and quench. Plusgas for a couple of days then use a hex socket (6 sided) to work backwards and forwards slowly.

I keep looking at the cheap stainless Chinese manifolds on ebay for around 150 a pair, as whilst its in bits??

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